This invention relates to a heavy-metal-modified noble metal catalyst on a fine-particle carbon support for the hydrogenation of aromatic halonitro compounds to aromatic haloamines characterized by good activity, selectivity and yields. In a further aspect, the present invention also relates to the process of producing said catalysts.
Aromatic haloamines are important starting materials for the production of pharmaceuticals, dyes, pesticides and herbicides. They are produced from the corresponding aromatic halonitro compounds, for example by catalytic hydrogenation. The hydrogenation is normally carried out using a solvent to convert the solid aromatic halonitro compounds into the liquid phase. Typical hydrogenation conditions are reaction temperatures of 25.degree. to 250.degree. C. and hydrogen pressures of 1 to 350 bar. Such hydrogenation reactions are well known in the art.
The catalytic hydrogenation is attended by the problem that not only is the nitro group reduced to the amino group, but the halogen atoms are also substituted by hydrogen atoms, thereby resulting in the formation of the corresponding hydrohalide acid. The acid formed, such as hydrochloric acid, causes corrosion in the reactor and, accordingly, has to be avoided by using a selective catalyst which hydrogenates only the nitro group.
Attempts have been made in the past to avoid the dehalogenation by using supported noble metal catalysts modified by suitable additives to improve their selectivity.
Suitable supports are carbon black, particularly acetylene black and active carbon. Platinum is mainly used as the noble metal component. Although palladium has better activity than platinum, it also has poorer selectivity. Selectivity can be improved by modification of the catalyst with sulfur or heavy metals.
Salek et al. (A. Salek, J.M. Berak, S. Tobola, W. Ormaniec, A. Teichert: Przemysl Chemiczny 58 (1979) 8; 425-427) report on studies to improve the activity and selectivity of a Pt/acetylene black catalyst in the hydrogenation of 3,4-dichloronitrobenzene. They arrive at the conclusion that good activity and selectivity can be obtained if the platinum content of the catalyst, based on the weight of the support, is at least 5% by weight, 15% by weight aniline is added to the reaction mixture and the catalyst is modified by 0.25 to 1% copper (where platinum and copper are simultaneously precipitated). Modification of the catalyst with copper produces an initially rapid reduction in the chloride concentration in the aqueous phase with increasing copper content and a slow reduction in activity. With 0.25% by weight copper, the chloride content is still above 2% by weight and only falls below 0.5% by weight where more than 0.5% by weight copper is used for modification. Modification of the catalyst with iron adversely affects selectivity. Salek et al. do not discuss the production of their catalysts in detail. The metal components were deposited onto the carbon black support by precipitation of the metal hydroxides from a solution of the metal salts with an NaHCO.sub.3 solution. The temperature prevailing during this process is not mentioned. The catalysts would appear to have been used for the hydrogenation without preliminary reduction.
Generally, the use of catalysts based on carbon black supports on an industrial scale is problematical on account of filtration problems during the wet chemical production of the catalysts and during their use in the process.
DE-OS 20 42 368 describes lead-, bismuth- or silver-modified Pt/C catalysts. The optimum platinum content of the catalyst is said to be 5% by weight. Unmodified catalysts lead to the complete elimination of chlorine in the hydrogenation of 2,5-dichloronitrobenzene. In contrast to the works of Salek et al., modification with 5% by weight copper still produced a chlorine elimination of 75% which is probably attributable to the fact that no aniline is used in DE-OS 20 42 368 to prevent the elimination of chlorine. Only the addition of 5% by weight lead, bismuth or silver reduced the elimination of chlorine to below 1%.
British patent application GB 2,024,643 describes a platinum catalyst on acetylene black which is modified with iron to improve its activity and to reduce dehalogenation and the formation of hydroxyl amine. The support material is optimally charged with 5% by weight platinum, a range of 2:1 to 16:1 being disclosed for the molar ratio of iron to platinum which corresponds to a ratio by weight of approximately 1:2 to 4:1. According to the British patent application, the carbon black supports are required to show minimal porosity and, accordingly, a relatively small specific surface of less than 300 m.sup.2 /g. A carbon black with a specific surface of only 35 m.sup.2 /g is used in the Examples. 70% of the particles of this support are smaller than 1 .mu.m in size. According to the application in question, a high activity of the catalyst is achieved by optimal deposition of the catalytically active components platinum and iron in fine-particle form on the fine-particle, non-porous carbon black particles. To this end, platinum and iron first have to be precipitated as oxides, hydroxides or carbonates from an aqueous solution of the salts of the metal components at a temperature of approximately 90.degree. C. in a suspension of carbon black and subsequently reduced with formaldehyde at room temperature.
If the platinum and iron are to be optimally distributed over the carbon black particles, it is crucially important that reduction is carried out at room temperature. Reduction at 90.degree. C. gives much poorer results.
The British patent application identified above also describes comparison tests with support materials having larger surfaces, namely an active carbon (support of control F) having a specific surface of 500 m.sup.2 /g and a carbon black support (support of control G) having a specific surface of 1050 m.sup.2 /g. 70% of the particles of the active carbon had a particle size below 30 .mu.m while 70% of the particles of the carbon black support were larger than 10 .mu.m in size. The catalyst on active carbon showed poor activity and resulted in a hydrogenation time of 133 minutes as against 84 minutes where the small surface carbon black support was used. Hydrogenation with the large surface carbon black support actually had to be terminated after 130 minutes because of excessive acid formation.
The articles "Platinkatalysatoren fur die Hydrierung von Nitrobenzolen (Platinum Catalysts for the Hydrogenation of Nitrobenzenes)" by J. Strutz and E. Hopf in Chem. Ing. Tech. 60 (1988) 4, 297-298 and "Selektive Hydrierung mit neuen Platinkatalysatoren (Selective Hydrogenation with New Platinum Catalysts)" by J.B.F. Anderson, K.G. Griffin and R.E. Richards in Chemie-Technik, 18 (1989), 5, 40-44 describe pure platinum hydrogenation catalysts on active carbon supports. Although J.B.F. Anderson et al. achieve good selectivities with their catalyst charged with 1% platinum, they do not mention the yields obtained. J. Strutz and E. Hopf also report on catalysts charged with 1% platinum. However, the yields they obtained are inadequate.
The known catalysts described above for the hydrogenation of aromatic halonitro compounds to aromatic haloamines are unsatisfactory. In some cases, they involve the use of very large quantities of noble metal, in addition to which their selectivity and yield are inadequate.
Accordingly, an object of the present invention was to provide a catalyst for the hydrogenation of aromatic halonitro compounds to form aromatic haloamines which would be distinguished from the known catalysts by a low noble metal demand and by improved activity and selectivity. Another object of the invention was to provide a process for the production of this catalyst.